One of the best known de-inking processes is the so-called washing process. In this process, the printed waste paper is slurried in water and the resulting suspension is successively washed. During pulping of the printed waste paper in water, the printed waste paper is crushed and fiberized in an alkaline medium, with the aid of heat, mechanical stirring, and with suitable de-inking compositions which remove the ink more or less thoroughly from the paper fibers.
The washing process involves a series of concentrating or thickening and dilution steps of the aqueous slurry obtained from the waste paper pulping and is designed to result in the formation of two aqueous suspensions; a more dilute water suspension containing the greatest possible quantity of the ink, and a more concentrated water suspension containing paper fibers that are as free as possible from the ink. This latter suspension is used in the manufacture of new paper. It may be said that dilution washing is the mechanical process of rinsing dispersed ink particles from pulp.
In practice, in the known washing processes, the more dilute water suspension containing the greatest possible quantity of the ink contains a considerable amount of paper fibers and fillers. Thus, the more dilute suspension must be suitably treated before being partially recycled or discharged as an effluent. In general, such treatment is accomplished by adding flocculants consisting of polyelectrolytes to the suspension which is then subjected to flotation in an acid or neutral medium in order to remove ink particles therefrom. See U.S. Pat. No. 3,354,028 to Illingworth. Proceeding in this manner, however, the fibers and mineral fillers are floated together with the ink, thereby causing an important part of the fibers and the mineral fillers to be lost, which otherwise would be useful for the preparation of new paper.
Thus, in summary, it may be said that with the de-inking compositions of the prior art and with the known de-inking washing processes the loss in recovered paper is in general relatively high. See Escher Wyss News, Vol. 44, 1971(1), pages 70 to 79.
Still another drawback of the known washing processes is that the floated mass, since it contains a considerable amount of fibers, has a considerable volume and, as such, is difficult to be disposed of.
Thus, the primary object of this invention is to provide an improved washing de-inking process that will enable one, with a limited consumption of reactants and with limited masses of processing water, to recover pulp having a high degree of brightness in a high yield.